Method of, and apparatus for, writing mask patterns on photographic material by means of light

ABSTRACT

A method of, and an apparatus for, writing mask patterns on a photographic material by means of light are described in which a spot of light is formed from light from a source of light by means of an opening which in principle is rectangular, the said light spot being swept over the photographic material in paths which correspond to the mask patterns to be written. It is shown that by separately controlling two boundaries which determine the length of the light spot in the direction of writing, for each infinitesimal element of a pattern element the time which elapses between the passage of the initial boundary of the light spot and the passage of the final boundary of the light spot over the infinitesimal element can be maintained constant.

United States Patent Klostermann et al.

[ Sept. 5, 1972 I METHOD OF, AND APPARATUS FOR, WRITING MASK PATTERNS ON PHOTOGRAPHIC MATERIAL BY MEANS OF LIGHT Inventors: Frits Theodoor Klostermann; An-

dreas Petrus Theodorus Hermanus [56] References Cited UNITED STATES PATENTS 3,330,182 7/1967 Gerber et al ..95/I 3,323,414 6/1967 I Ritchie et a1 .....355/43 X Primary ExaminerSamuel S. Matthews Assistant Examiner-Kenneth C. Hutchinson Attorney-Frank R. Trifari ABSTRACT A method of, and an apparatus for, writing mask patterns on a photographic material by means of light are described in which a spot of light is formed from light from a source of light by means of an opening which in principle is rectangular, the said light spot being swept over the photographic material in paths which correspond to the mask patterns to be written.

It is shown that by separately controlling two boundaries which determine the length of the light spot in the direction of writing, for each infinitesimal element of a pattern element the time which elapses between the passage of the initial boundary of the light spot and the passage of the final boundary of the light spot over the infinitesimal element can be maintained constant.

11 Claims, 4 Drawing Figures METHOD OF, AND APPARATUS FOR, WRITING MASK PATTERNS ON PHOTOGRAPHIC MATERIAL BY MEANS OF LIGHT The invention relates to a method of writing mask patterns on a photographic material by meansof light, in which method there a spot of light is formed from the light of a source of light by means of an opening which in principle is rectangular, said light spot being moved over the photographic material along paths which correspond to the mask patterns to be written.

The invention also relates to an apparatus for writing mask patterns on a photographic material by means of light. The term light is used herein to mean an electromagnetic oscillation having a wavelength between 0.2 pm and 2 um.

ln the'manufacture of integrated circuits and largecapacity stores photomasks are used. Since the masks have to satisfy exacting requirements in respect of their dimensions, in the manufacture of photomasks patterns of these masks are first drawn to anenlarged scale with a'high'degree of accuracy and subsequently reducedto the desired size by photographic means. 1

In a known method, see for example the article Mask Manufacture for integrated Circuits in The Western Electric Engineer, Vol. XI, No. 4, pages 35 37, the desired pattern is first cut by a numerically controlled drawing machine in the upper layer of a twolayer plastic foil of, say, 120 X 120 cm? After the upper layer has been locally peeled off, i.e. after the opaque material within the contours cut has been removed, the

pattern is reduced, say, 30 times with a fixed camera arrangement. Then the resulting mask pattern is further reduced, for example 10 times, in a step-and-repeat camera, a large number of images being successively formed in different areas of a photographic plate, so that a large number of equal mask patterns .of the desired size are produced on this plate.

The only non-automatized stage of this method is the local peeling of the opaque upper layer of the plastic foil, whichoperation demands a high degree of accuracy and takes much time. Peeling requires far more time than drawing. The above process further requires a large room.

By writing the initial mask patterns by means of light instead of cutting them from a plastic foil the following advantages are obtainable:

l. the non-automatized peeling stage may be dispensed with, so that:

a. much time can be saved in the manufacture of photomasks,

b. no errors due to incorrect peeling of the plastic foil occur in the finished photomasks;

2. the size of the initial large written pattern may be greatly reduced with an equal or even improved final accuracy, so that a. no intermediate reduction is required,

b. the time required for writing the first large pattern can be reduced by a factor of, say, 10. g

The pattern elements of the mask pattern to be written must have a prescribed width throughout their length. The location of the transition from black to transparent along a path depends upon the location of the transition between light energy and no light energy, but also, because this transition is indistinct in practice, upon the amount of the light energy. The point at which in a transition from transparent to black a density 2 is obtained depends upon the point at which in this transition an exposure energy associated with the density 2 is fOUlldplf the location of the transitions from black to transparent along a path is to be controlled, the exposure energy must be controlled. The exposure energy depends upon the intensity of illumination on the photographic material and upon the exposure time.

For writing by means of light one may start from either of two principles. According to the first principle, the photographic material may be exposed by means of a light spot which is stationary relative to the material or by means of a quasi-stationary light spot, by exposing with a light pulse of very short duration while moving the material. According to the second principle the photographic material may be exposed by means of a light spot which essentially moves relative to the material.

ln exposure by means of a stationary light spot there are in principle no difficulties in maintaining a constant exposure energy. Exposure in the stationary condition is described inter alia in .Advances in Computer Generation of Master Artwork for Microminiature Circuits, S.C.P. and Solid State Techn. (Vol. 10, July 7 1967, pages 31-34). This method has the disadvantage that the obtainable latitude in the geometry is very limited. ln'view of the time required to build the entire mask pattern, in this method the geometry must be built from a comparatively limited number of rectangles.

With continuous exposure during movement of the light spot relative to the photographic material, the exposure time is given by r l'lv, where l is the length of the lightspot in the direction of writing and v is the writing velocity. Without special expedients, T will assume an undesirable value owing to the accelerations and decelerations which occur at the beginning and at the end respectively of a line being written. The consequent undesirable effects can be eliminated partly only by adaptation of the intensity of illumination.

The paper Microcircuit Photomasks from Automatic Techniques" in IEEE Transactions on Electron Devices, vol. ED-l2, No. 12, Dec. 1965 p. 638643, describes a method of writing photomasks by means of a moving light spot in which a solution of this difficulty is given. A light spot of a length which in principle is constant is swept over the photographic material at a velocity which in principle is uniform. At a point at which no exposure is desired, the light spot is blocked by a shutter. At a point at which a pattern element is to begin or to end the shutter is moved so that the transition light-dark in the light spot will be temporarily stationary relative to the photographic material. Thus, a constant exposure energy per unit area is obtainable throughout the entire length of a pattem element to be system as a whole relative to the photographic material. Matching errors may readily slip in during the com plicated controlling process and they will result in incorrect locations of the beginning and the end of a pattern element. Further, a large disadvantage is that accelerations and decelerations of the spot of light relative to the photographic material occur outside the line segment, and this takes additional time.

The invention provides a simple method of writing mask patterns by means of light, in which method the aforementioned disadvantages do not occur. The invention is characterized in that the length of the light spot in the direction of writing is determined by two boundaries which are separately controlled so that for each infinitesimal element of a pattern element to be written the time elapsing between the passage of the initial boundary of the light spot and the passage of the final boundary of the light spot over the infinitesimal element is the same, irrespective of the accelerations or decelerations in the movement of the light spot relative to the photographic material. Thus the exposure time is equal for each infinitesimal element. With the use of a constant intensity of illumination the exposure energy also will be the same for each infinitesimal element of the pattern elementbeing written.

According to a further feature of the method according to the invention, the light spot width at right angles to the writing direction is determined by two adjustable transverse boundaries which lie, at least substantially, in the same plane as the initial and final boundaries.

The initial and final boundaries and the transverse boundaries may materially be spaced from the photographic material by a very small distance. However, it is more advantageous to dispose the boundaries at some distance from the photographic material-and to form reduced images of them by means of a lens system.

In anvapparatus for carrying out the method according to the invention, the initial and final boundaries are constituted by two knives which are arranged in the path of the light from the light source to the photographic material present during the writing process, during which process one of the knives is stationary with respect to an optical system which forms the light spot, and the other is movable with respect to this optical system.

In a preferred embodiment of an apparatus according to the invention, the transverse boundaries are constituted by one of a number of transparent areas of different lengths provided in the radial direction in an opaque disc which is rotatable about an axis at right angles to the disc.

In a further preferred embodiment of an apparatus for carrying out the method according to the invention there is arranged in the path of the light between the boundaries and the lens system a mirror system mounted in a rotatable drum so that the axis of rotation substantially coincides with the optical axis of the lens system. This permits writing in any desired direction so that there is complete freedom of geometry. The mirror system may comprise a reflecting prism and a plane mirror.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIGS. 1 and 2 illustrate the principle of the invention,

FIG. 3 is a front elevation of the apparatus according to the invention, and

FIG. 4 is a top plan view of the apparatus according to the invention taken on the line 22' of FIG. 3.

Referring now to FIG.'1, a photographic plate on which the mask patterns are to be written is designated 1. The x direction is the direction in which the light spot is swept over the photographic material. The photographic plate itself is displaceable in a direction y at right angles to the x direction, i.e. at right angles to the plane of the drawing in FIG. 1. The light spot is bounded in the writing direction by knives A and B. These knives"A and B are note material knives but they are the images of material knives A and B produced by a lens system 7 (see FIG. 3).

The knife image A is moved according to a function:

If now the knife image B is moved relative to the knife image A with a constant time lag 1' and hence according to a function:

only has to satisfy the requirement of being continuous and of ascending monotonically in t. For an arbitrary point x=a, with (l s a s x,, we have at the beginning of the exposure:

fUIm ilminy) [I and for the end of the exposure:

g end) f(lend (I From l and (2) it follows:

rm) beuiuuinu T Consequently, the exposure time (tgzposure temi tbeginn1ny) is constant for each 0 s x s x, and equal to 7.

Each image element can be written starting from the stationary condition, which is important for the total writing time required and for the freedom of geometry.

The method according to the invention may be carried out by means of an apparatus as shown in FIG. 3. In the apparatus of FIG. 3, light from a source of light 4 is concentrated on to one of a number of slits in a disc 6 by a condenser 5. After passing through this slit the light beam passes through the opening between the material knives A and B located close to the disc 6. The knives A and B and the part of the respective slit located between the knives are imaged by an objective 7; A is the image ofA and B is the image of B.

FIG. 4 is a top plan view of the disc 6 and of the knives A and B viewed according to the line ZZ' of FIG. 3. The disc is provided with radial slits S of different lengths. By rotating the disc, one of the slits is lined up with the knives A and B. The length of this slit determines the width of the image element to be written;

By suitable choosing the distance between the knives A and B and the lens system'7 on the one hand and that between the lens system 7 and the photographic plate 1 on the other, a highly defined reduced image of the opening between the knives A and Bis produced on the photographic plate 1.

The assembly of the projection system (4, 5, 7) and of the rotatable disc 6 may bemoved relative to the photographic material in the .writing direction 1;, for example by moving the photographic material in its plane. The knife A is stationary with respect to the optical axis and hence follows the movements of the projection. system. The knife B, however, is movable relative to the projection system-and hence to the knife A also. The desired absolute movement of the knife B and its desired movement relative to the knife A are computed in a device, not shown, and the signals resulting from the computation are applied to the device controlling the movement of the knife B, which device is designated 8 in FIGS. 3 and 4. FIGS. 3 and 4 show the case in which writing is effected in a direction from left to right. Obviously, writing may alternatively .be in a direction from right to left. In this event, the knife B will be stationary with respect to the projection system andthe knife A will be movable relative to this system. if desired, both knives A and B may be controlled. A mirror system mounted in a rotatable drum 9 is arranged between the knives A and B and the objective. The axis of rotation substantially coincides with the optical axis of the lens system 7. The mirror system comprisesa reflecting prism 10 and a plane mirror 11. The incident beam is first reflected by a reflecting surface 12 to the plane mirror 1 1. This mirror reflects the beam to a reflecting surface 13, and this surface directs the beam on to the lens system 7. When the drum 9 and hence the mirror-system. 10, llare rotated, A' and B will be rotated. Thus, writing may be effected in any desired direction. I g

In a practical embodiment of the apparatus according to the invention, an area of 200 X 200 mm could be written with a writing speed of 10 mm per second and -with accelerations and decelerations in the writing manufacture of photomasks for integrated circuits and large-capacity stores. The method and apparatus according to the invention may also be used for direct writing of mask patterns on a semi-conductor material,

without the intermediate step of manufacturing of photomasks.-So in the method of discretionary wiring approach, whereby on a chip of semi-conductor material, in which chip electrical components such as transistors already have been formed, conduction patterns interconnecting the components may be directly formed, the method and apparatus also may be used. What is claimed is: v l. A method of writing a mask pattern on photora hic terial, om risin e ste s o r vidin a ou ce of ii ght, prrijectiiig th li g ht thr bug s it me ns 'rate in a path which corresponds to the mask pattern to be written, and moving the other boundary in correspondence with said one'boundary after a constant time delay so thatthe time elapsed between the forward and rear edges of the incident light past each infinitesimal element of the photographic material is the same.

2. A method as claimed in claim 1, further including the step of varying the width of the light incident on the photographic material by means of two adjustable transverse boundaries.

3. A method as claimed in claim 1, wherein said source of light is substantially constant.

4. A method as claimed inclaim 1, wherein said boundaries are separately controllable.

5. An apparatus for writing a mask pattern on photographic material, comprising a source of light, a slit means, an optical. system for projecting the light through said slit means and onto the photographic material, said slit'means having two oppositely located boundaries which determine the length of the lightincident on the photographic material in the direction of writing, means to move the one of said boundaries at a variable rate in a path which corresponds to the mask pattern to be written, and means to move the other boundary in correspondence with said one boundary after a constant time delay so that the time elapsed between the forward and rear edges of the incident light past each infinitesimal element of the photographic material is the same.

6. An apparatus as claimed in claim 5, wherein the one boundary is stationary with respect to said optical system and the other boundary is moved relative to said optical system.

7. An apparatus as claimed in claim 6, further comprising an opaque disc having a plurality of transparent slots for determining the width of the light incident on the photographic material, said slots being of different lengths and being radially disposed.

8. An apparatus as claimed in claim 7, wherein said optical system comprises a mirror system mounted in a rotatable drum, the axis of the drum substantially coinciding with the axis of the projected light.

9. An apparatus as claimed in claim 8, wherein the mirror system comprises a reflecting prism and a plane mirror.

10. An apparatus as claimed in claim 5, wherein said boundaries are separately movable.

11. An apparatus as claimed in claim 5, wherein said source of light is substantially constant. 

1. A method of writing a mask pattern on photographic material, comprising the steps of providing a source of light, projecting the light through a slit means and onto the photographic material with an optical system, said slit means having two oppositely located boundaries which determine the length of the light incident on the photographic material in the direction of writing, moving one of said boundaries at a variable rate in a path which corresponds to the mask pattern to be written, and moving the other boundary in correspondence with said one boundary after a constant time delay so that the time elapsed between the forward and rear edges of the incident light past each infinitesimal element of the photographic material is the same.
 2. A method as claimed in claim 1, further including the step of varying the width of the light incident on the photographic material by means of two adjustable transverse boundaries.
 3. A method as claimed in claim 1, wherein said source of light is substantially constant.
 4. A method as claimed in claim 1, wherein said boundaries are separately controllable.
 5. An apparatus for writing a mask pattern on photographic material, comprising a source of light, a slit means, an optical system for projecting the light through said slit means and onto the photographic material, said slit means having two oppositely located boundaries which determine the length of the light incident on the photographic material in the direction of writing, means to move the one of said boundaries at a variable rate in a path which corresponds to the mask pattern to be written, and means to move the other boundary in correspondence with saId one boundary after a constant time delay so that the time elapsed between the forward and rear edges of the incident light past each infinitesimal element of the photographic material is the same.
 6. An apparatus as claimed in claim 5, wherein the one boundary is stationary with respect to said optical system and the other boundary is moved relative to said optical system.
 7. An apparatus as claimed in claim 6, further comprising an opaque disc having a plurality of transparent slots for determining the width of the light incident on the photographic material, said slots being of different lengths and being radially disposed.
 8. An apparatus as claimed in claim 7, wherein said optical system comprises a mirror system mounted in a rotatable drum, the axis of the drum substantially coinciding with the axis of the projected light.
 9. An apparatus as claimed in claim 8, wherein the mirror system comprises a reflecting prism and a plane mirror.
 10. An apparatus as claimed in claim 5, wherein said boundaries are separately movable.
 11. An apparatus as claimed in claim 5, wherein said source of light is substantially constant. 